An exemplary modular color printer includes a number of tandemly arranged electrophotographic image-forming modules (see for example, U.S. Pat. No. 6,184,911). Such a printer includes two or more single-color image forming stations or modules arranged in tandem and an insulating transport web for moving receiver members such as paper sheets through the image forming stations, wherein in each module a single-color toner image is transferred from an image carrier, i.e., a photoconductor (PC) or an intermediate transfer member (ITM), to a receiver held electrostatically or mechanically to the transport web, and the single-color toner images from each of the two or more single-color image forming stations are successively laid down in registry one upon the other to produce a plural or multicolor toner image on the receiver. The receiver carrying the unfused plural image is then sent to a fusing station where the multicolor toner image is thermally fixed or fused to the receiver using heat and/or pressure in known fashion.
In a digital electrophotographic copier or printer, a uniformly charged PC surface may be exposed pixel by pixel using an electro-optical exposure device including light emitting diodes, such as for example described by Y. S. Ng et al., Imaging Science and Technology, 47th Annual Conference Proceedings (1994), pages 622–625.
A widely practiced method of improving toner transfer is by use of so-called surface treated toners. As is well-known, surface treated toner particles have adhered to their surfaces sub-micron particles, e.g., of silica, alumina, titania, and the like (so-called surface additives or surface additive particles). Surface treated toners generally have weaker adhesion to a smooth surface than untreated toners, and therefore surface treated toners can be electrostatically transferred more efficiently from a PC or an ITM to another member.
As disclosed in the Rimai et al., U.S. Pat. No. 5,084,735, and in the Zaretsky and Gomes, U.S. Pat. No. 5,370,961, use of a compliant ITM roller coated by a thick compliant layer and a relatively thin hard overcoat improves the quality of electrostatic toner transfer from an imaging member to a receiver, as compared to a non-compliant intermediate roller.
As is well-known, a toner image may be formed on a PC, preferably a photoconductive roller, by the sequential steps of uniformly charging the PC surface in a charging station using a corona charger, exposing the charged PC to a pattern of light in an exposure station to form a latent electrostatic image, and toning the electrostatic image in a development station to form a toner image on the PC surface. The exposing pattern of light has a one-to-one correspondence with an original image, which original image is for reproduction or visualization as the toner image. The electrostatic image may be developed using discharge area development (DAD) in which charged toner particles having a same polarity as that of the surface corona charges are attracted to exposed areas of the PC. Alternatively, charged area development (CAD) may be used in which the toner particles have an opposite polarity to the corona charges. Preferably, discharge area development is used in a printing machine using a digital exposure device. After development, the toner image can be transferred in a transfer station directly to a receiver member, e.g., a paper sheet, or it may first be transferred to an ITM, preferably an intermediate transfer roller, and subsequently transferred to the receiver.
A background area is defined as a macroscopic area on an imaging member, which macroscopic area corresponds to an area in the original image in which there is no image information, e.g., in which the density is substantially zero. Such an original image can be a color separation image. Ideally there is no toner deposition in a background area. In order to keep a background area on the PC substantially free of toner, the development electric field in the development station typically has a polarity such as to locally repel nominally charged toner particles in the background area. This reverse bias field in a background area is determined by the bias potential applied in the development station.
In charged area development (CAD) of a toner image, a Dmax area having a maximum amount of toner lay down corresponds to an area having substantially zero photoexposure, i.e., at a potential V0, whilst a background area in CAD corresponds to an area of high photoexposure in which the surface potential has been discharged by the exposure device to a voltage below a threshold voltage for toning, which threshold voltage has a magnitude smaller than the magnitude of the applied bias potential, Vb. Thus in an imaging area the onset of toning at threshold occurs despite a repelling electric field. In CAD practice, a background area is an area in which the amount of photoexposure is sufficient to drive the magnitude of the surface potential below the magnitude of Vb, i.e., intermediate between Vb and the residual voltage which is the limiting surface potential produced by very large photoexposures.
On the other hand, in discharged area development (DAD) the areas of maximum photoexposure by the photodischarge device correspond to Dmax areas, and a DAD background area is an area of low photoexposure (which can be substantially zero photoexposure). As in a CAD process, in a DAD process the threshold surface potential for toning in an imaging area occurs when there is a reverse bias electric field. As is well known, in the DAD case the threshold surface potential for toning has a magnitude intermediate between V0 and the applied bias potential, Vb.
The optimal situation for both CAD and DAD processes typically occurs when a development sump contains fresh developer, i.e., when the toner particles are optimally triboelectrically charged by carrier particles that have had relatively little or no wear, so that the background areas remain substantially clean during the development process.
The ability to charge fresh toner particles in a toning station decreases with developer age, and also depends on the dispense rate of fresh toner added per unit time to the developer so as to match the toner takeout rate. Developer wear generally manifests itself by slowed charging rate and/or by inability to charge freshly added toner to a desired level. As a result, a developer mixture contains some weakly charged or uncharged toner particles, and in an extreme case an aged developer can contain wrong-polarity toners having a net charge opposite to that of suitably charged toner particles. Such weakly charged, uncharged, or wrong-polarity toners tend to be disadvantageously deposited in background areas, and can cause unacceptably large background toner concentrations.
As the number of weakly charged, uncharged, or wrong-polarity toners in a developer increases as the developer ages, the result is a movement of the toe of a development curve to lower development voltages. Ultimately, such toner particles are deposited in background areas despite the reverse biasing which locally provides a repelling electric field.
Exemplary FIG. 1A demonstrates this situation for the case of charged area development using for example a magnetic brush apparatus, where for ease of illustration only, the PC surface is taken to be charged positively (giving positive surface potentials) and negative toner particles develop the electrostatic image. Thus in this exemplary CAD process the surface is initially charged to a positive voltage V0 (zero photoexposure) and photodischarged to some potential Vexp, as depicted schematically by the curve labeled “1” in the left hand graph of FIG. 1A where Vexp is plotted as a function of photoexposure. At very large photoexposure the surface potential levels off at a residual potential, Vr. The graph depicted on the right hand side of FIG. 1A schematically shows a corresponding plot of developed mass per unit area (DMA) as a function of the development voltage when using a fresh developer, where DMA is a quantitative measure of toner coverage. For zero exposure, the coverage is greatest with developed mass per unit area equal to DMAmax, and for exposure corresponding to threshold, Eth, the DMA is zero. In the present example, the development voltage is defined as (Vexp−Vb) and is controlled by the bias level Vb, with Vexp>Vb. The threshold photodischarge voltage for toning, Vth, determines the onset of the toe of the development curve, which occurs for a threshold development voltage Va=(Vth−Vb). Va is negative in this example, i.e., toner particles are typically deposited even when a weak reverse bias electric field exists during development, owing to the presence of non-electrostatic forces, as is well known. However, for all potentials below Vb, the development electric field is in a direction to attract wrong-sign toners (i.e., toners having a net positive charge in the present case). The difference of potential (Vth−Vr) is commonly known as the background latitude window. Using properly charged toner particles, a development voltage, Vbkg, produced by a photoexposure, Ebkg, results in a clean background, as does any other development voltage which is more negative than Vbkg.
Exemplary FIG. 1B (corresponding to FIG. 1A) schematically shows developed mass per unit area plotted as a function of development voltage in the threshold or toe region. The lower curve labeled “acceptable developer” is essentially the same as the threshold portion of the curve in the right hand portion of FIG. 1A. The curve labeled “marginal developer” shows the effect of a certain degree of developer aging, where the threshold development voltage has shifted from Va to a more negative value, Va′. Ultimately, with an even older developer, the threshold development voltage will become more negative than Vbkg, which means that toner will be disadvantageously deposited in background areas.
Exemplary figures (not shown), corresponding to FIGS. 1A and 1B can be constructed for DAD development using a magnetic brush apparatus (see above), where Vb is set close to V0 rather than close to Vr, Vth is positioned between V0 and Vb, and the DMA declines as photoexposure increases.
Returning to FIG. 1A, the photodischarge curve labeled “2” schematically shows the effect of photoconductor aging or fatigue. Larger exposures are required to produce a given amount of photodischarge, and the residual voltage is higher. This reduces the magnitude of the background latitude window, with the result as sketched that even with fresh developer the background areas may not be clean for an exposure equal to Ebkg. Thus fatigue of a photoconductor can produce a degradation of the background areas similar to that caused by aging of the developer. Thus early warning of photoconductor fatigue can be obtained by use of the subject invention. As will be seen below, a result of photoconductor fatigue can in certain cases be distinguished from a result caused by aging of a developer.
The Bean et al., U.S. Pat. No. 4,124,287, discloses forming an image wise non-uniform charge pattern on a photoconductor surface and a contacting the surface with uncharged marking particles. Briefly, a patch containing a pattern of alternating charge density preferably of high spatial frequency can be produced by any of a number of ways as disclosed in U.S. Pat. No. 4,124,287, e.g., via photoexposure of a thin, charged, photoconductive layer or by moving a photoconductor surface under an AC corona charger having a slit for passage of the corona ions. Contacting a resulting charge pattern with uncharged particles results in polarization of the particles in the highly non-uniform electric fields associated with the charge pattern, with the result that the polarized particles are attracted to the surface by the dielectrophoretic forces. Calculations of such polarization forces exerted on a spherical toner particle above a periodically charged photoreceptor are given for example by I. Chen, Journal of Photographic Science and Engineering Vol. 26, page 153 (1982). A discussion of a similar phenomenon for magnetic toners in a highly uniform magnetic field is given for example in an article by J. Bares, Journal of Photographic Science and Engineering Vol. 28 (3), pages 111–118 (1984).
An optoelectronic circuit for measuring the optical density of a toned test patch in comparison with an untoned area is disclosed by the DeWolf et al., U.S. Pat. No. 4,750,838. A phototransistor is used to measure light reflected from the test patch. An exemplary circuit is described for measuring optical densities in a range 0.5–1.5. However, as mentioned (column 6, line 66 of the DeWolf et al. patent) the technique is capable of measuring optical densities in a range 0.0–1.5.
The Bares patent, U.S. Pat. No. 4,924,263, discloses use of a synchronous detection circuit to measure a surface concentration (or coverage) of magnetized or magnetizable toner particles on a photoconductor, and to generate there from a resulting control signal. A patch image in the form of a bar pattern made from parallel equi-spaced strips is created on the photoconductor surface via corona charging, photoexposure, and development in standard fashion. The bars of the bar pattern are formed perpendicular to the direction of motion of the photoconductor surface. As the bar pattern moves past a magnetic read head adjacent to the PC surface, an alternating magnetic field is sensed by the read head so as to produce an alternating voltage which when passed through a low pass filter (having a time constant, T0) produces a read head AC voltage signal. Simultaneously, a beam of light is bounced off the moving bar pattern and the reflected beam is passed into an optical detector, which monitors changes in reflectivity as the bars move past. An alternating reflection intensity sensed by the optical detector produces an alternating voltage which when passed through a low pass filter (having a similar time constant, T0) produces a reference AC voltage signal. After any suitable pre-amplication(s) the read head AC voltage signal and the reference AC voltage signal, which are in phase with one another, may be represented respectively as (V0) G(t/T0) Cos(ωt) and (Vor) G(t/T0) Cos(ωt), where ω is 2π cycles per unit time and G(t/T0) is a gate function which relates to the number of bars scanned. The magnitude of the voltage V0 is proportional to the coverage of toner in the bar pattern. The read head AC voltage signal and the reference AC voltage signal are multiplied together in a signal multiplier device so as to produce a multiplied signal of the form (V0)(Vor) G(t/T0) Cos2(ωt), which is identically equal to 0.5(V0)(Vor) G(t/T0)(1+Cos(2ωt)). After passage of this multiplied signal through a low pass filter, the DC component is obtained as an output control signal equal to 0.5(V0)(Vor) G(t/T0). This output control signal, being proportional to the voltage reading of the magnetic read head, can then be used to regulate various processing stations in a reproduction machine using a magnetic ink character recognition format.
As disclosed in Bares, U.S. Pat. No. 4,999,673, a processing station in an electrophotographic printing machine, is controlled by monitoring a toned test patch on a photoconductor, using an infrared densitometer. The test patch, e.g. in the form of equi-spaced strips, preferably (but not necessarily) perpendicular to the direction of motion of the photoconductor, is located in an interframe (interimage) area on the photoconductor. The test patch can be representative of a portion of an image frame, such that an output signal from the infrared densitometer can be used to regulate a processing station, e.g., so as to adjust set points for charging, exposing or toning in the image frame.
A densitometer for optically detecting a reflectance signal which is inversely proportional to a coverage of a color toner in a test area on a photoconductor surface is disclosed in Genovese, U.S. Pat. No. 5,204,538. The densitometer is used to measure reflection optical density sequentially from two different sources of radiation, i.e., using alternately operating near infrared light sources and a single photodiode detector to detect the rays from each source as reflected from the test area located in an interframe (interimage) area on a photoconductive belt of an electrophotographic color printing machine. The test area is created in a similar manner as that of an adjacent image area on the photoconductor, i.e., using similar set points for charging of the photoconductor and developing of the respective electrostatic images. The photoexposure device for exposing the test area is separate from the photoexposure device for exposing the adjacent image area. One of the light sources in the densitometer, e.g. a primary LED, is focused on to the test area so that reflected light from that portion of the photoconductor not covered by toner (R) plus the scattered light (S) from the toner particles in the test area are simultaneously detected by the photodiode detector, i.e., as (R+S). The second source is preferably an array of more than one secondary LED such that each secondary LED is located off axis from the primary LED, and only scattered reflected light (S) reaches the detector. Thus alternating signals of (R+S) and S are obtained, so that R can be extracted electronically. The quantity R is inversely proportional to the toner coverage being measured in the test area. The densitometer is capable of measuring toner coverages over a wide gamut, from low to high coverage (see FIG. 3 of the Genovese patent). In view of the AC signal produced by the alternation of the primary and secondary light sources, the resulting AC voltage signal is conducive to synchronous detection and integration circuitry, which can be used to extract minute signals from a noisy background.
The above-cited patents are not primarily concerned with detection and/or monitoring of background toner particles per se. For example, Bares, U.S. Pat. No. 4,924,263, does not specifically disclose application to low coverages of toner in background areas, and discusses only magnetized or magnetizable toners. However, low coverages of toner particles can be measured according to U.S. Pat. Nos. 4,750,838 and 5,204,538.
Apparatus for measuring background toner on a receiver sheet is disclosed in Bares, U.S. Pat. No. 5,214,471. A raster input scanner such as a CCD device is used to scan a toned area in a print (after transfer from a photoconductor to the receiver) and thereby to measure background toner concentration thereon, essentially by counting background toner particles on a pixel-by-pixel basis. Suitable pattern recognition algorithms can be used to distinguish indicia such as text and/or objects having a toner lay down greater than a pre-selected threshold lay down. The number of particles counted by such a CCD device produces a control signal which can be used to regulate various processing stations in an electrophotographic machine, or which can be used to generate a service call.
While the apparatus of Bares, U.S. Pat. No. 5,214,471, is capable of measuring low coverages of background toner, it is relatively expensive and requires high-speed data processing so as to be able to count all relevant pixels in an image on a moving photoconductor surface.